(1) Field of Invention
The present invention relates to the detection of combustion system malfunctions in gas turbines, and more particularly, to a method for the early detection of gas-turbine combustor damage due to flashbacks so as to minimize subsequent hot-gas path damage and leakage of combustible gas into the turbine enclosure.
(2) Background Information
In recent years there has been an increase in the regulatory requirements for low emissions of pollutants, such as Oxides of Nitrogen (called NOx) from gas-turbine power plants. One method for controlling gas-turbine emissions is the use of a combustor design which limits the formation of pollutants in the burning zone by using lean-premixed combustion technology.
A gas-turbine combustor is essentially a device used for mixing large quantities of fuel and air and burning the resulting mixture. Gas-turbines with combustion systems designed to reduce NOx emissions to levels below 40 ppm without water or steam injection employ a combustion process in which fuel is uniformly mixed with air prior to the combustion process. In the premixing zone, ignition of the fuel and air occasionally occurs. This event, regardless of its cause, is usually called a "flashback". Due to the design of most premix systems, the combustion of fuel and air in the premix section usually causes considerable damage to components. For various reasons, it is often not practical to design a low NOx combustor to operate satisfactorily with flame in the premix section. To prevent damage in the event of a flashback, it is necessary to quickly shut-off the premixer fuel and inject the fuel into another fuel nozzle passage, if provided, or simply trip the machine.
Flashback damage has historically been detected using NOx emission and exhaust temperature spreads as indicators. When a flashback occurs, NOx increases and exhaust temperature spreads often, but not always, increase. The NOx increase is typically proportional to the severity of the flashback. On the other hand, the exhaust temperature spread change can vary, either decreasing or increasing, depending upon the state of the combustors, which suffer flashback, prior to the flashback event. The unpredictable behavior of exhaust temperature spreads, coupled with the emissions data scatter, has made it difficult to determine whether or not a flashback has occurred using NOx and exhaust spread indicators. NOx or spread changes alone are insufficient to indicate a flashback event. Methods which rely on changes in NOx and exhaust profile over sequential instants of time to determine if a flashback has occurred are ineffective because changes in NOx and exhaust profile can occur during loading. Therefore, any loading may appear to be a flashback using this method.